1. Field of the Invention
The present invention relates to a dielectric ceramic composition used as a dielectric layer of an electronic device, such as a multilayer ceramic capacitor, a production method thereof and an electronic device comprised of the dielectric ceramic composition as a dielectric layer.
2. Description of the Related Art
A multilayer ceramic capacitor as an example of electronic devices is produced, for example, by printing internal electrodes in a predetermined pattern on a ceramic green sheet formed by a predetermined dielectric ceramic composition, alternately stacking the results to form as one body, and co-firing a thus obtained green chip. Since internal electrode layers of the multilayer ceramic capacitor are formed to be one body with ceramic dielectric by firing, materials not reacting with the ceramic dielectric has to be selected. Therefore, in the related art, it has been necessary to use expensive precious metals, such as platinum and palladium, as the material composing the internal electrode layer.
In recent years, however, dielectric ceramic compositions using inexpensive base metals, such as nickel and copper, have been developed and a drastic reduction of costs has been realized.
Also, in recent years, demands for downsizing of electronic devices have become stronger along with electronic circuits becoming higher in density, and multilayer ceramic capacitors have rapidly become downsized with a larger capacity. To attain a downsized multilayer ceramic capacitor with a larger capacity, a method of making dielectric layers thinner and a method of heightening specific permittivity of a dielectric ceramic composition included in dielectric layers, etc. have been generally used. However, when dielectric layers are made thin, an electric field imposed on the dielectric layers becomes stronger at the time of applying a direct current voltage, consequently, there has been a disadvantage that a change of the specific permittivity with time, that is, a change of the capacity with time becomes remarkably large.
To improve a change of the capacity with time under a direct current electric field, there has been proposed a method of using dielectric particles having a small average crystal grain diameter as dielectric particles to be included in the dielectric layers (for example, refer to the Japanese Unexamined Patent Publication No. H08-124785). The Japanese Unexamined Patent Publication No. H08-124785 discloses a dielectric ceramic composition having a specific composition, wherein an average crystal grain diameter of the dielectric particles is 0.45 μm or smaller. However, the dielectric ceramic composition described in the article has too low specific permittivity to respond to downsizing and obtaining of a larger capacity.
Also, the present inventors have disclosed an electronic device in the Japanese Unexamined Patent Publication No. 2005-29423, such as a multilayer ceramic capacitor, having improved specific permittivity and accelerated lifetime of insulation resistance (IR) by dividing rare earth elements to two groups based on a value of effective ionic radius for coordination number 9, setting an adding quantity of elements belonging to one group and an adding quantity of elements belonging to the other group to be in a specific range, and setting a ratio of the adding quantities to be in a specific range.
However, along with rapid downsizing of electronic devices, multilayer ceramic capacitors are also required to be furthermore downsized and to have a larger capacity. As explained above, as a method of downsizing a multilayer ceramic capacitor and increasing the capacity, a method of making the dielectric layers thinner and a method of improving the specific permittivity are used. When making the dielectric layers thinner, a change of specific permittivity with time and other characteristics (a temperature characteristic of capacitance and IR accelerated lifetime, etc.) are required to be equivalent to those before making the layers thinner or even improved. Accordingly, to attain downsizing of a multilayer ceramic capacitor and to increase the capacity, the dielectric layers are required to be thinner while maintaining various characteristics (a change of specific permittivity with time, a temperature characteristic of capacitance and an IR accelerated lifetime, etc.) and to improve the specific permittivity. However, in specific examples of the Japanese Unexamined Patent Publication No. 2005-29423, a thickness of one dielectric layer is 3.5 μm, but when the layer is furthermore made thinner (for example, 1.5 μm), there have been cases where the specific permittivity declined and a temperature characteristic of the capacitance declined. Therefore, even when the dielectric layers are made furthermore thinner, it has been required to improve the specific permittivity while maintaining a temperature characteristic of the capacitance, IR accelerated lifetime and other characteristics preferable.